ABSTRACT Cannabis use among young adults is widespread and national studies indicate increases in recent years in prevalence of cannabis use, including daily use. As of November 2018, 10 U.S. states and the District of Columbia have legalized cannabis for non-medical (?recreational?) use for adults aged 21 and older. Washington State was on the forefront of these changes, voting to approve legalization in 2012 and effectively implementing the legislation two years later. Although a major argument for legalization was its potential to decrease the human, social, and economic costs related to criminal justice involvement, an un-intended detrimental consequence might involve its impact on cannabis and other substance use (SU). Importantly, the impact of policy implementation may be felt differently across the state. Local jurisdictions in many communities passed ordinances that impose restrictions on the cannabis market. Further, in municipalities where no bans are in place, the geographic distribution of stores is uneven, with cannabis retail stores often concentrating in socio-economically disadvantaged neighborhoods. Research on the evaluation of the effects of legalization on cannabis use is only in its infancy; little is known about the etiology of SU and related health risk behaviors, such as driving while intoxicated, and the role of community- and individual level cannabis- specific risk factors in the context of legalized cannabis. Moreover, it is unclear how relationships among use of different substances may change during the course of implementation or developmentally. The salience of known risk factors may also change during implementation, by context or over the course of young adulthood. This project capitalizes on unique and rich data we collected with funding from the WA State Department of Social and Health Services. The WA State Young Adult Health Survey (YAHS) is an accelerated longitudinal cohort study of young adults ages 18-29 from 2014-2018 with the first data collection occurring before any legal cannabis stores opened and additional cohorts added in each of the four subsequent years. The survey data will be linked with a cannabis policy database that denotes local variation in cannabis context, including regulations and restrictions on sales, and with data on density of stores and neighborhood disadvantage. This project aims to examine effects of cannabis legalization on SU and related health risk behaviors among young adults in WA State. We will also assess within-state variation in SU outcomes as it relates to community-level factors such as city and county policy, cannabis and alcohol retail outlet availability, and neighborhood disadvantage. Moreover, we will examine the trajectories of SU and their interplay over the course of young adulthood and the role of the community-level factors in the etiology of young adult SU and related outcomes in the context of legalized cannabis. Understanding these effects will inform (1) planning of prevention and intervention efforts aimed at improving health outcomes and reducing problem behaviors in the changing cannabis policy context; and (2) regulatory provisions for the existing and new policies related to cannabis.